Today, composite dental fillings are used increasingly and have completely displaced the use of amalgams in the area of the front teeth, mostly for aesthetic reasons. As regards the region of the lateral teeth, however, composite dental fillings have not gained wide acceptance because of the absence of certain properties which are required for clinical uccess.
Among the properties required for composite dental fillings are freedom from pores, X-ray opacity, resistance to abrasion and complete edge seal. There have been many attempts to achieve these properties in order to make available the undisputed aesthetic advantages associated with composite dental fillings as well as the possibility of mercury-free fillings for the dental side regions.
The problem relating to freedom from pores has been solved quite satisfactorily by the introduction of light-hardening composites, whereby manual mixing is no longer required. X-ray opacity is achievable by the use of special glass fillers doped with heavy metals such as barium or strontium. To improve resistance to abrasion and thereby minimize material wear of the filling substance, various improvements have been suggested which do not give excellent results, but which have, nevertheless, proved adequate. For example, the tearing-loose of filler particles from the resin matrix giving rise to rough surfaces, and hence rapid abrasion, has been reduced by treating the filler particles with special bonding agents (for instance silane). This procedure allows the formation of a chemicl bond between the resin matrix and the filler to resist large stresses. Additionally the particle size of the filler has been optimized. Thus, these new and so-called "hybrid composites" contain fillers with an average particle size in the range of about 0.5 and 5 microns(.mu.) and a proportion of highly dispersed silicic acid. Such particles can neither project much above the filling surface nor, if tearing loose, leave a large crater behind that would weaken the entire filling. Nevertheless they exhibit a high Young's modulus and adequate hardness.
Despite the above improvements, the problem of obtaining a complete edge seal still exists. This intactness is required so that microorganisms are prevented from entering the edge gap between the filling and the tooth and possibly causing the formation of secondary caries. The edge gap is caused by the polymerization shrinkage. All known composites have shrinkage values of about 2.5% by volume and larger. Prevention of the formation of an edge gap has already been attempted by bonding the filling to the hard substance of the tooth using an adhesive. So far this has been successful only for relatively small fillings, with shrinkages of a few microns which are wholly within the enamel area. However, for larger fillings, such an adhesive technique is not entirely satisfactory since the bonding means for dentine cannot resist the shrinkage forces of larger fillings, and for normal oral techniques, the restored work becomes rapidly leaky. While improvements have been achieved using various applicat.ion techniques such as build-up in layers and shrinkage vector reversal (hardening from the side of the tooth), they have nevertheless proved inadequate for clinical success.
A new preparation for improving the sealing of the edge consists in a composite inlay which is similar to the long known gold and ceramic inlays of dentistry. First the dentist takes an impression of the cavity to be filled using an elastomeric impression material. The cavity is then temporarily closed and the patient is discharged. The dental technician then makes a hard plaster model of this impression mold. The hard plaster model is divided into model segments, and thereupon the modeling with composite takes place. A light-hardening substance is used, which is polymerized by direct illumination on the stump model. To achieve better physical properties, especially good abrasion resistance and Young's modulus, the stump model after removal of the inlay is improved in an oven for instance 15 minutes at 100.degree. C. or 5 to 10 minutes at 120.degree. C. The removed inlay is finished and polished can be inserted into the patient after the dentist has taken out the stop-gap filling and cleaned and dried the cavity.
In the first instance, the above known procedure allows the use of an optimally shaped and hardened material which is polymerized completely before being inserted and so will not shrink any further. Accordingly an optimal edge seal is also obtained in conjunction with the cement used to bond the inlay to the tooth. Further it is possible to achieve good contact points, ideal occlusion-shaping and the prevention of excess, especially at the near cervical edge. However this procedure is very complex and time-consuming. The patient must call at least twice on the dentist and a stop-gap filling is mandatory. Additionally, there is the danger of mistakes occurring during the long finishing stage, whereby the work would have to be repeated. Furthermore, the procedure cannot be shortened because it has been impossible so far to post-process a pre-polymerized inlay because this inlay cannot be removed from the cavity without destroying its shape.